Question: $ A = \left[\begin{array}{r}-1 \\ -1 \\ 3\end{array}\right]$ $ D = \left[\begin{array}{rr}-1 & 3\end{array}\right]$ What is $ A D$ ?
Answer: Because $ A$ has dimensions $(3\times1)$ and $ D$ has dimensions $(1\times2)$ , the answer matrix will have dimensions $(3\times2)$ $ A D = \left[\begin{array}{r}{-1} \\ {-1} \\ \color{gray}{3}\end{array}\right] \left[\begin{array}{rr}{-1} & \color{#DF0030}{3}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ A$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ A$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ A$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rr}{-1}\cdot{-1} & ? \\ ? & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ A$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rr}{-1}\cdot{-1} & ? \\ {-1}\cdot{-1} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ A$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rr}{-1}\cdot{-1} & {-1}\cdot\color{#DF0030}{3} \\ {-1}\cdot{-1} & ? \\ ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{-1}\cdot{-1} & {-1}\cdot\color{#DF0030}{3} \\ {-1}\cdot{-1} & {-1}\cdot\color{#DF0030}{3} \\ \color{gray}{3}\cdot{-1} & \color{gray}{3}\cdot\color{#DF0030}{3}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}1 & -3 \\ 1 & -3 \\ -3 & 9\end{array}\right] $